The systems and techniques described herein were made in the performance of work under a NASA contract, and are subject to the provisions of Public Law 96-517 (35 USC 202) in which the Contractor has elected to retain title.
This application relates to techniques and devices for optical sensing and optical detection.
Optical sensing generally uses an optical probe beam to interact with a material to be detected. This interaction between the optical probe beam and the material modifies some aspect of the optical probe beam. A portion of this modified beam, such as the scattered light, the reflected light, or the transmitted light, may be collected and measured to obtain certain information of the material. For example, the optical intensity, phase, spectrum, polarization, or direction of the collected light may be measured either individually or in combination with other parameters to determine the certain information of the material.
Optical sensing may be used to achieve a number of advantages. For example, the sensing can be non-invasive and does not alter the material to be measured under proper operating conditions. The spectrum of the probe beam may be controlled to selectively interact with only certain optical transitions in specified particles, molecules, or atoms in the material. Optical sensing may also be used to achieve high detection sensitivity and to detect minute amount of a particular material.
This disclosure includes optical sensing techniques and devices based on whispering-gallery-mode micro resonators or cavities. An optical probe beam is evanescently coupled into at least one whispering gallery mode of such a resonator. A sample material to be measured may be filled within the resonator or surrounded outside the resonator to interact with and modify the whispering gallery mode or geometry of the resonator. The evanescent field outside the resonator is detected or measured to detect a change caused by the modification. This change is then processed to extract information about the sample material. This change may be reflected as, e.g., a temporal change in the mode structure during a transient period, attenuation in the evanescent field, a frequency shift in the whispering gallery mode and its evanescent field, or a change in efficiency of the evanescent coupling of the probe beam into the resonator or coupling of the energy in the whispering gallery mode out of the resonator.
In one implementation, the quality factor of the resonator for a particular mode may be measured to extract the information about the sample material by, e.g., measuring the spectral bandwidth of the mode or decay time of the mode. In he another implementation, the frequency shift is measured. In yet another implementation, the intensity of light coupled out of the resonator is measured. The resonator may also be actively controlled, e.g., by using a control signal to adjust its dimension, this control signal required to maintain certain desired conditions can be measured before and after the sample is introduced. The change in the control signal, e.g., its magnitude, may be used to extract information on the sample.